Molecular mechanisms controlling seed set in cereal crop species under stress and non-stress conditions

1. Introduction

Food security is of global concern, especially in developing countries (Rosegrant and Cline 2003). The six most widely grown crops in the world are maize, rice, wheat, soybean,barley, and sorghum. These crops are grown for seed on more than 40% of global cropland area, and account for 55% of non-meat calories and over 70% of animal feed. Among all seed crop species, cereal crops are most important for humans, which provide over 70% of the food consumed by humans. Though the seed yield per land area has increased during the last decades, 20–40% of the world’s potential crop production still is lost because of weeds, pests, diseases, and abiotic stress factors according to the FAO (2015). Even in major cereal crops producing countries, rice, wheat, and maize still have a 10–15% yield gap between actual and potential seed yields (Tollenaar and Lee 2002; Cassman et al. 2003; Grassini et al. 2011).However, there can be about 50% yield gap in specific crop planting areas. For example, the average maize yield in the US is 10.7 Mg ha–1, but the maximum yields exceed 18.8 Mg ha–1. Maize yield in China has the potential to increase dramatically due to the introduction of hybrids that are better adapted to denser planting conditions, mechanization, and improvements in seed technology (Li et al. 2011). Yield potential is defined as the yield of a cultivar grown under non-limiting abiotic and biotic conditions in an environment to which it is adapted. Causes for the yield gap are unfavorable abiotic or biotic growth conditions, limited water and nutrient supply, or exposure of plants to abiotic and/or biotic stress.Increased seed yield of cereal crops is predominantly the result of improved stress tolerance (Duvick 1997; Fasoula and Fasoula 2002).

In cereal crops, the process of seed formation can be divided into three stages: seed set, seed growth, and seed maturation (Ruan et al. 2012). Seed set is established during and soon after fertilization with two genetically identical male sperm cells. One male sperm cell fuses with the haploid egg cell, resulting in a diploid embryo. The other sperm cell fuses with two (female) haploid central cells,resulting in a triploid endosperm. This double fertilization event initiates seed development. Embryo and endosperm are surrounded by a maternally derived seed coat, which provides a shield for the developing seed (Köhler and Makarevich 2006). The newly formed seed then undergoes cell expansion and accumulation of storage products, mainly proteins, starch, and oils, which are typical features of growth and maturation stages (Weber et al. 2005).

During the process of seed formation in cereal crops,seed set features a transition from fertilized ovules to seed,which has a profound impact on later seed developmental stages, and determines seed number by determining cell numbers, and seed yield potential (Fig. 1). Seed set is more sensitive to internal or external stresses compared to later stages of seed development or vegetative growth (Boyer and McLaughlin 2007; Suwa et al. 2010; Zinn et al. 2010).Stresses include insufficient supply of nutrients, drought,heat, cold, high plant density, presence of weeds, plant diseases, and insect pests, which often induce floral or seed abortion and irreversible seed yield losses.

Maize and rice are the two most important cereal crops in the world, which have different floral architectures(monoecious vs. hermaphrodite species, respectively) and photosynthetic pathways (C4 vs. C3, respectively). Here,we focus on discussing genetic mechanisms controlling seed set in maize and rice at four developmental stages:(1) development of floral structure; (2) formation of viable gametes; (3) double fertilization; and (4) seed development and abortion.

2. Development of male and female floral architectures

Fig. 1 The major development stage and seed yield component in cereals.

Seed set is influenced by the floral architecture in cereal crops, which depends on the number and arrangement of floral branches (Vollbrecht et al. 2005). Variation in branching patterns lead to diversity in inflorescences and seed number (Satoh-Nagasawa et al. 2006). Maize possesses two types of inflorescences: the tassel and the ear. The tassel is a terminal, staminate inflorescence. The ear is a pistillate inflorescence produced on a lateral branch,which is a major yield trait controlled by the developmental fate of axillary shoot meristems (Satoh-Nagasawa et al.2006). The units of rice floral structure are spikelets and florets. Spike (an unbranched in florescence in which stalkless spikelets are arranged on an elongated axis)initiation and spikelet (contains one or more florets enclosed by two glumes) formation is the first phase of reproductive growth. This process provides the developmental basis for spike differentiation, which contributes to altering final seed number and yield potential (Sreenivasulu and Schnurbusch 2012). In the past decades, various major genes or QTL affecting the development of floral architecture have been cloned. In maize, inflorescence branching is regulated by three RAMOSA genes. RAMOSA1 (RA1) is a plant specific epidermal patterning factor-like protein (EPF-like protein),regulating the branching architecture of maize in florescence.In RA1 mutants, both the tassel and ear become more branched. RAMOSA2 (RA2) encodes a lateral organ boundaries-domain protein (LOB-domain protein) whose RNA is expressed at the edge of the bract and meristem early in inflorescence development. RAMOSA3 (RA3)encodes a trehalose 6-phosphate phosphatase and is expressed in discrete domains subtending axillary in florescence meristems. RA2 and RA3 act upstream of RA1, and RA3 may act in parallel with RA2 (Vollbrecht et al.2005; Satoh-Nagasawa et al. 2006). In rice, grain number 1a (Gn1a), encoding a cytokinin oxidase in rice (OsCKX2),wealthy farmers panicle1 (WFP1), encoding OsSPL14,and dense and erect panicle1 (OsDEP1), encoding a truncated phosphatidylethanolamine-binding protein are three important genes controlling the development of floral organs. Reduced expression of OsCKX2 causes cytokinin accumulation in in florescence meristems and increases the number of reproductive organs, resulting in enhanced grain yield (Ashikari et al. 2005). In contrast, increased expression of OsDEP1 enhances meristematic activity,resulting in a reduced length of in florescence internodes,an increased grain number per panicle and consequently,increased grain yield. Increased expression of OsDEP1 reduces the level of OsCKX2 expression, suggesting that OsDEP1 acts upstream of OsCKX2 to control cytokinin homeostasis in panicle meristems (Huang et al. 2009).Increased expression of OsSPL14 in the reproductive stage promotes panicle branching and grain yield, and also controls shoot branching at the vegetative stage (Miura et al. 2010). Although those genes are involved in different pathways to control the development of floral architecture,they play important roles in regulating seed number and arrangement.

四川省家庭服务行业协会还在筹备中，至今没有颁布行业规范和统一的服务质量标准，也没有制定统一的、符合家政服务特点的劳务派遣合同。

在过去近六十年的编程历史中，编程语言的抽象级别不断提高，人们都在努力让编程语言更有表现力，这样我们可以用更少的代码完成更多的工作。笔者经过查阅资料发现：如今影响力较大的趋势主要有三种，“声明式的编程风格”(包括“领域特定语言”及“函数式编程”)、“动态语言”(其最重要的方面便是“元编程”能力)以及多核环境下的“并发编程。此外随着语言的发展，原本常用的“面向对象”语言，“动态语言”或是“函数式”等边界也变得越来越模糊，例如各种主要的编程语言都受到函数式语言的影响。

3. Formation of viable (male and female)gametes

图6中平均用户传输速率随信道资源的增多而增大，同时四种算法间差异增大， CIDG因完全掌握环境信息，理论上当迭代次数趋向无穷大时，能找到系统最优解.相比之下，SLG模型平均减少了近(M-1)*N/M的交互量，传输速率仍能逼近CIDG，进一步说明了SLG算法的有效性.

基于前述制剂生产特点，如没有信息系统的帮助，单靠一两个统计和财务人员手工操作，制剂成本核算盲目追求理论上的按品种的产品成本等完全成本核算方式，不但核算工作量太大，而且容易产生偏差误导决策。随着信息技术的不断发展，信息系统在医院财务管理中的应用越来越广泛，医院制剂的生产和核算信息化必将成为改善制剂管理的有力手段。一方面，制剂部门配备专门的制剂库存管理软件，实现对制剂物资的高效管理；另一方面，系统软件逐一计算每种药品的材料及费用，从而实现单个制剂成品的成本核算。

Flowering time is also an important selection criterion in cereal breeding, which not only influences plant demand for resources and a plant’s ability to capture resource for growth(Dong et al. 2012), but also influences floral development and seed set. Among cereal crops, winter wheat relies on vernalization for flowering. Rice is a short day plant, and flowering time is day length sensitive. Maize undergoes the transition to flowering after a fixed number of leaves has been produced (Bortiri and Hake 2007; Itoh et al.2010). Tropical maize is an exception, which is photoperiod sensitive for flowering time, and unadapted to temperate latitudes. However, the basic genetic mechanisms controlling flowering time are largely conserved among cereals (Lagercrantz 2009; Song et al. 2010). For example,the FRUITFUL1 (FUL1)/VERNALIZATION1 (VRN1) protein is associated with specifying competence to flower initiation and transition after a cold treatment (vernalization) in wheat(Danyluk et al. 2003; Murai et al. 2003; Loukoianov et al.2005). In maize, ZMM4 is a MADS-box gene in the FUL1 family that regulates floral transition and in florescence development, which can be activated after floral transition in early developing inflorescences. Over-expression of ZMM4 leads to early flowering in transgenic maize (Danilevskaya et al. 2008). In rice, overexpression of OsMADS18 can induce early flowering, accelerate the formation of axillary shoot meristem (Fornara et al. 2004). Although evidence from various studies supports a complex gene network responsible for floral transition and floral development in cereals (Yano et al. 2001; Izawa et al. 2003; Boss et al.2004; Bernier and Perilleux 2005), the function of VRN1 in wheat, ZMM4 in maize, and OsMADS18 in rice is similar to that APETALA1 (AP1) in Arabidopsis. AP1 plays a central role in the transition from floral induction to flower formation(Wellmer and Riechmann 2010).

During flower induction and in florescence development,drought stress leads to failure of panicle exertion and anther dehiscence in rice and sorghum (Ekanayake et al. 1990;Craufurd and Peacock 1993; Wopereis et al. 1996), which leads to a substantial reduction in seed set. In maize,seed abortion is highly dependent on the timing of water stress. Low water availability before pollination results in seed abortion even if sufficient water was available at the time of pollination. Drought stress inhibits maize pistillate flower development, pistil integrity, ovule functions, and grain weight (Westgate 1994), while timing of male in florescence development and pollen shed is less affected, resulting in increased anthesis-silking intervals (ASI). An extended ASI reduces the chance that female spikelets will be selfpollinated and contribute to incomplete pollination (Fuad-Hassan et al. 2008), and thus increases the risk of yield reduction (Monneveux et al. 2006; Brekke et al. 2011).

Fig. 2 The sugar regulated mechanism for pollen and ovary fertilization. CWIN, cell wall invertase; PCD, programmed cell death.

Ovary abortion has similar molecular mechanisms as pollen sterility (Ruan et al. 2010). In maize ovaries, phloem-imported sucrose supplies carbon for starch accumulation in ovary walls and to generate high glucose concentrations by CWIN in pedicels (Mclaughlin et al. 2004). Upon imposing a water deficit 5 days before an thesis, sucrose import is blocked due to inhibited leaf photosynthesis and remobilized ovary wall starch reserves. However, these become rapidly depleted, if drought persists for several days.Concomitantly, CWIN activities and glucose concentrations decrease, leading to ovary abortion and yield loss. A CWIN glucose signaling pathway is the primary genetic mechanism controlling maize ovary abortion (Mclaughlin et al. 2004). Glucose can repress the programmed cell death (PCD) pathway and promote cell division of filial cells(Ruan et al. 2012). Under stress conditions, photosynthetic activity is reduced (Schussler and Westgate 1991; Chaves et al. 2003), which results in decrease of glucose levels,followed by pollen sterility in anthers and activation of a PCD pathway leading to seed abortion in ovaries (Fig. 2).In summary, glucose is a key factor for pollen fertility and ovary development and, therefore, seed set in cereal crop species (Fig. 2).

4. Double fertilization

For successful seed set, pollen and ovaries in cereal crops must remain viable, pollen tubes must grow properly and release two sperm cells into the ovular embryo sac for double fertilization to produce embryo and endosperm.In maize, heat stress (＞38°C) can lead to reduced pollen germination ability and pollen tube elongation, resulting in reduced seed set and seed production (Fischer 1985;Schoper et al. 1987). Increased temperature during the mid-anthesis period decreased the grain number per ear in spring wheat (Ferris et al. 1998), demonstrating heat sensitivity of fertilization and seed set. High-temperature stress at flowering reduces spikelet fertility in rice. Sterility is caused by poor anther dehiscence and low pollen grain production, and hence a low number of germinating pollen grains on the stigma (Matsui and Omasa 2002; Prasad et al. 2006).

Many studies also showed that viability of pollen is related to its water content and the humidity of atmosphere. Pollen needs strong protection (anther) against desiccation in time. Low water content of pollen can affect pollen growth speed and survival. However, anthers will not open at high air humidity, which can lead to premature loss of viability(Aylor 2003).

对于不是急症手术的肝胆部手术患者,对于严重的肝病患者,手术之前需要纠正临床及实验室的各种异常为主,加强营养,改善凝血功能,血浆蛋白低者术前积极准备必要时给予输血浆或白蛋白,有腹水者进行腹腔穿刺,改善呼吸功能。根据手术切除范围,做好术中用血的准备。

In cereal crops, pollen and ovary development greatly depend on an adequate import or utilization of photoassimilates,mainly in the form of sucrose and starch. For pollen to be viable, it is necessary to synthesize sufficient starch,cellulose, and callose, which are main components for building internal pollen wall tubes. Starch, cellulose, and callose are all polymerized from glucose in α-1,4, β-1,4, and β-1,3 linkages, respectively (Kudlicka et al. 1997; Cai et al.2011). Glucose can be derived from sucrose hydrolysis and starch degradation by cell wall invertase (CWIN) and α-amylase, respectively (Fig. 2). The CWIN-mediated metabolic pathway is the main route to produce glucose.Reduction in CWIN expression or sucrose content leads to male sterility and seed abortion in wheat under drought stress (Koonjul et al. 2005) and in rice under cold stress(Oliver et al. 2007). Sucrose-rich pollen survives for a longer time than sucrose-poor pollen in maize and other crops(Hoekstra et al. 1989; Buitinik et al. 1996; Pacini 1996).Thus, sucrose supply and CWIN activity are key to male fertility and seed set.

In summary, pollen development is more prone to heat stress than that of ovaries. However, fertility of ovaries has a greater influence on seed set than that of pollen under drought stress in cereals (Boyer and McLaughlin 2007;Barnabás et al. 2008). Cereal breeding must develop cultivars tolerating multiple types of stresses impacting seed production (Tester and Bacic 2005). Low ASI under abiotic stress conditions is, therefore, important breeding goal.

In nature, self-incompatibility (SI) is a genetic mechanism to prevent self-fertilization by inhibiting the germination of pollen on stigmas, or the elongation of pollen tubes.Several grass species share a common incompatibility mechanism controlled by two unlinked loci, S and Z (Li et al. 1997). SI promotes cross-fertilization to produce heterozygous and vigorous plants. SI can be inactivated by high temperatures leading to pseudocompatibility in perennial ryegrass and Ipomoea fistulosa (Prabha et al.1982; Wilkins and Thorogood 1992). Pseudocompatibility will lead to self-pollination, and inbred offspring and thus directly and indirectly reduces seed set and seed yield.Although SI has not been reported in cereal crops including maize, rice, wheat, and barley, breeders have tried to utilize SI in F1 hybrid breeding systems (Do Canto et al. 2016).SI is also widespread in grasses and other plant families.

In addition, water stress during flower induction and in florescence development leads to a delay in flowering(anthesis), or even to complete inhibition (Mahalakshmi 1985; Wopereis 1996; Winkel 1997). High temperatures during floret formation cause complete or partial abortion(Sanini and Aspinall 1982), in which the main effect of heat stress is reduction of kernel number (Fischer 1985).

5. Seed development and abortion

Seed development is regulated by the interplay of phytohormones. Gibberellic acid (GA3) increases source strength by improving photosynthetic efficiency by influencing photosynthesis related enzymes (i.e., Rubisco, fructose-1,6-biphosphatase, and sucrose phosphate synthase), leaf area, light interception, and phloem loading (Iqbal et al.2011). GA3 also increases sink strength by promoting cell division, general growth, and carbohydrate import by inducing sucrolytic activities, namely CWIN, a key enzyme in the regulation of phloem unloading (Roitsch and González 2004). GA3 signaling is involved in metabolic adjustment for maintaining source-sink relations, increasing the efficiency of assimilate production and transport under limiting environmental conditions (Achard et al. 2006). However,most studies indicate that GA3 signaling in response to stress reduces growth to allow plant adaptation and survival.Cytokinin (CK) levels play a key regulatory role for plant growth and survival. Optimal CK levels are necessary to increase not only leaf longevity and photosynthetic capacity (source strength) but also growth of sink organs(sink strength) under abiotic stress (Ha et al. 2012; Zalabák et al. 2013). Constitutive over-expression of the CK-degrading enzyme cytokinin oxidase (CKX) or inhibition of the CK-biosynthetic IPT1, IPT3, IPT5, and IPT7 genes resulted in CK deficiency and enhanced drought and salt stress-tolerant phenotypes in A. thaliana (Nishiyama et al.2011). However, in rice, reduced expression of OsCKX2 causes cytokinin accumulation in in florescence meristems and increases the number of reproductive organs, resulting in enhanced grain yield (Ashikari et al. 2005). Auxins influence carbon partitioning and stimulate mobilization of carbohydrates in leaves and the upper stem and increase translocation of assimilates towards sink organs (Smith and Samach 2013). Auxins regulate the activity of CWIN and thus sucrose allocation in sink organs (Albacete et al.2008). ABA has been implicated in male sterility of tomato,rice, and wheat (Morgan 1980; Morgan and King 1984;Westgate et al. 1996). Exogenous ABA application inhibits the activity of invertases and monosaccharide transporters to reduce sucrose content, leading to pollen sterility and PCD in barley (Parish et al. 2012). However, some studies in cereals reported positive correlations between grain ABA content and efficient seed filling by optimizing faster remobilization events from stem reserves (nonstructural carbohydrates), a critical factor in sustaining grain filling and grain yield under drought stress (Yang et al. 2004). Ethylene is often regarded as a growth inhibitor (Pierik et al. 2007).Male gametophyte development is susceptible to ethylene at the stage of pollen mitosis. Ethylene has been used to manipulate the development of pollen grains by application of an ethylene inhibitor, to promote dry mass accumulation and concentrations of starch and reducing sugars in anthers of basal spikelets leading to improved seed set in rice (Naik and Mohapatra 1999). In summary, seed set is controlled by a phytohormone interplay, controlling the balance between source and sink strength under both optimal and stress conditions.

Seed development greatly depends on adequate supply of photosynthetic products and nutrient use efficiency.Water deficiency results in inhibition of photosynthesis,and reduces nutrient supply to generative organs. At the same time, water shortage limits N uptake from soil and decreases the nitrate concentration in xylem, affecting ABA accumulation in leaves, which leads to altered ear structure and yield reduction (Carcova et al. 2000). In addition, high planting density decreases light penetration into the canopy, reducing photosynthetic capacity (Hammer et al. 2009), which then affects the final kernel number and characteristics (Borras et al. 2003). Many studies reported that grain number and weight both decreased in maize as plant density increased, suggesting a complex interaction between the sink and assimilate supply (Sangoi et al. 2002; Hashemi et al. 2005; Lashkari et al. 2011).Under full sunlight, plants are exposed to relatively equal fluxes of light with wavelengths of 600–700 nm (red light)and 700–800 nm (far-red light) (Holmes and Smith 1977).When plant density increases, red light is intercepted,while far-red light is largely reflected, creating a far-red light enriched environment. Under this condition, a series of morphological changes take place, including increased plant height, decreased leaf blade area (Smith 1995), decreased stem diameter (Lashkari et al. 2011), and delayed pollen shed and silking (Tokatlidis and Koutroubas 2004). Plant growth rate is reduced during reproductive stages (Rossini et al. 2011), resulting in partitioning of more assimilates towards vegetative instead of reproductive growth (Kebrom and Brutnell 2007). In addition, the axillary position of ears in maize is subject to apical dominance, assimilates are partitioned to the shoot rather than the ear under high planting density (Sangoi et al. 2002), which increases the risk that ears will have poor or not seed set.

Biotic stress occurs as a result of damage to plants by bacteria, viruses, fungi, parasites, insects, and weeds.Plants are under constant assault by pathogens and competitors during their development (Pimentel 1991),and evolved a myriad of defenses to meet requirements for cellular maintenance, growth, and reproduction (Tian et al.2003; Berger et al. 2007). Although defense response to different biotic stressors is highly variable, the transcriptional response to biotic stress is highly coordinated, which means that biotic stress globally down-regulates photosynthesis genes (Zou et al. 2005; Berger et al. 2007; Bilgin et al.2010). In order to trigger defenses to dissuade biotic stresses, plants will newly allocate resources from growth to defense by a reduction of photosynthetic capacity in leaf tissues (Nabity et al. 2009), which leads to reduced carbon assimilation, resulting in poor seed set. Abiotic and biotic stresses factors are most likely to occur simultaneously under field conditions, which has a devastating impact on crop productivity. Combined effects of these stresses are greater than the effects of single stress factors alone(Suzuki et al. 2014). Moreover, studies still found that abiotic stresses can reduce the resistance of plants to biotic stresses (Szittya et al. 2003). Groszmann et al.(2015) suggests that a suppression of defense response gene activities are important for generating the hybrid vigor phenotype. So over-expression of defense and stress response genes will reduce seed production or hybrid vigor in cereal crops.

σi为风险资产i的标准差，wi为投资组合中投资于风险资产i的比例，设风险资产i的非系统性风险的组合标准差为σB，显然有：

In contrast to sexual seed formation, apomixis is an evolutionary mechanism of seed formation without fertilization, which can occur by various genetic regulators reported for more than 300 species in 30 out of 460 angiosperm families (Kandemir and Saygili 2015). Although these different mechanisms have not yet been fully elucidated, it is now generally agreed that apomixis is a qualitative trait controlled by a few genes (Hanna et al. 1998;Barcaccia and Albertini 2013). Fertilization-independent seed (FIS) mutants in A. thaliana have the ability to form seed-like structures without fertilization (Chaudhury et al.1997), which give significant clues about the genetic mechanism of apomixis. Three important genes have been identified and cloned in A. thaliana, which includes the FIE gene (Fertilization Independent Endosperm, encoding a WD type POLYCOMB protein) (Ohad et al. 1996), the MEDEA gene (encoding a SET domain type POLYCOMB protein) (Grossniklaus et al. 1998), and the FIS2 gene(Fertilization Independent Seed 2, encoding a zinc finger protein) (Chaudhury et al. 1997). In recent years, apomixis has been seen as a way of multiplying superior genotypes clonally by seed. However, apomixis has not been found in cereals (Bashaw 1980; van Dijk et al. 2016). Recently,a apomixis gene, ASGR-BABY BOOM-like in Pennisetum(PsASGR-BBML) has been cloned and introduced into rice and the resulting plants developed embryos from egg cell without fertilization. But the negative feature is low seed set in articifially induced apomictic plant (Ozias-Akins and Connor 2015). Although transfer of apomixis to crop species through wide crosses has many additional hurdles so far(Kandemir and Saygili 2015), understanding the function of natural apomixis genes and the introduction of apomixis into cereal crop species would significantly alter breeding strategies of agricultural crop species.

6. Future prospects for studying seed set in cereal crop species

Seed set is a complex quantitative trait, which is affected by various environmental factors. However, it is critically important for cereal crop yield. Substantial progress has been made in understanding the biology of seed set.Several genetic pathways have been identified to control seed set in cereal crops. Moreover, the benefits of whole-genome sequence information generated in rice, maize, and Arabidopsis can be extended using synteny and collinearity relationships among cereal crop species. For example,rice chromosome 1 shows regions of sequence similarity with chromosomes 3, 6, and 8 in maize (Salse et al. 2004)and homoeologous group 3 chromosomes in bread wheat(Moore et al. 1995), where some QTLs for grain yield and other agronomic traits have been mapped (Dilbirligi et al.2006). Gn1a (OsCKX2) in rice might correspond to these maize and wheat QTL, and orthologous CKX genes might regulate yield in other cereal crops (Ashikari et al. 2005).Once identified through synteny, these genes for seed set can be manipulated to improve grain yield. However, it is still unclear how genetic and non-genetic factors coordinate control of seed set in cereal crops. As we know, seed yield improvement in cereal crops may have been the result of an improved genetic×agronomic management interactions, rather than the result of either genetic and/or agronomic improvement per se (Tollenaar and Lee 2002).For example, grain yield of USA maize hybrids from the 1930s to 1990s did not differ when plants were grown under low-density conditions, where competition among plants for soil moisture, soil nutrients, and incident solar radiation was negligible (Duvick 1997). Thus, yield improvement in cereal crops has been associated with increased stress tolerance. Under environmental stress, plants reallocate resources to defense of biotic and abiotic stresses. Today,the main challenge is to decrease the yield gap between the potential seed yield and realized seed yield. Future research is required on tradeoffs between growth processes and defense response to stress. More generally, there is a need to establish a genetic framework for seed set and to better de fine and prioritize the molecular mechanisms and factors controlling seed set.

Acknowledgements

Authors would like to thank the National Key Research and Development Program of China (2016YFD100103), the Major Science and Technology Projects in Henan Province,China (161100110500, 151100111000), the Science Foundation for the Excellent Youth Scholars of Henan Academy of Agricultural Sciences, China (2016YQ04), the International Cooperation Project in Henan Province, China(162102410034), as well as USDA’s National Institute of Food and Agriculture (IOW04314, IOW01018), the RF Baker Center for Plant Breeding and K. J. Frey Chair in Agronomy at Iowa State University for funding this work.

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